In tire retreading operations, the worn tread is stripped or buffed from the crown of the tire carcass and new tread rubber is bonded in place. In one type of operation, the new tread rubber is in a cured state when placed on the carcass and a bonding layer of, for example, gum rubber is placed between the crown of the carcass and the cured tread strip. The bonding layer is cured to secure the tread to the carcass. In one type of curing operation, the retread tire assembly is placed in an envelope that is then evacuated. The assembly and envelope are placed in a chamber for heat and pressure treatment to cure the bonding layer.
In another type of operation, the tread rubber is in AN uncured state and is applied to the carcass crown and cured in place. The assembly of carcass and uncured tread rubber is placed in a curing press having a bladder that fits within the interior cavity of the tire assembly and heating means to heat the bladder and the area surrounding the tire tread.
A curing chamber is a large pressure and temperature controlled vessel having a capacity for several tire assemblies. One type curing chamber for curing 25 tire assemblies has an interior volume of 905 cubic feet. Typically, to heat the air in a curing chamber, heated steam or oil is forced through a heat exchanger in the curing chamber, or electrical resistance elements in the curing chamber are used. Each of these has deficiencies.
Steam requires a steam boiler and associated piping and control elements that are expensive to install and maintain. A heated oil system is also expensive to install and maintain, and requires special care to prevent fire hazards. Both steam and heated oil depend on heat transfer to heat the air in the curing chamber. Electrical resistance heats the air directly, but is expensive to operate.
U.S. Pat. No. 6,267,084 (hereinafter “the '084 patent”), owned by Applicants' assignee and incorporated herein by reference for all purposes, describes an apparatus for curing retread tire assemblies that uses heated, pressurized water as the heating medium for a curing chamber. Such apparatus is less expensive to install and maintain than either a steam or oil system. The apparatus is less expensive to operate than electrical resistance heating elements. Furthermore, the heat transfer capacity needed for tire curing operations can be achieved using water as a heat transfer medium.
In one embodiment described in the '084 patent, the apparatus includes a curing chamber having an interior space in which retread tire assemblies may be placed for curing. The curing chamber has a heat exchanger in the interior space. An electric or gas powered vessel to heat water is connected in a closed circuit to the heat exchanger. The air in the curing chamber is circulated by a fan to ensure uniformity of the air temperature throughout the chamber and to facilitate heat transfer from the heat exchanger. The vessel has the capacity to heat water to at least 290° F. (143° C.). The vessel preferably has a volumetric capacity of approximately 20 gallons of water. A pump, preferably a centrifugal pump, is disposed in the closed circuit to pump heated water between the vessel and the heat exchanger under pressure. The pump provides a flow rate of 20 to 50 gallons per minute, and preferably about 25 to 35 gallons per minute. For this embodiment of the '084 patent, a control valve maintains a flow of heated water at a constant rate to the curing chamber to meet heating demand. A temperature sensor senses the temperature in the curing chamber, measuring the air temperature or the temperature at the retread tire assembly, and provides feedback to the control valve for controlling the flow of heated water.
When using water as the heating medium in a system such as that described in the '084 patent, if proper system pressure is not maintained throughout the curing process, water in the system will convert to steam, which can cause cavitations in the pump and subsequent mechanical failure. Furthermore, during startup, the system can undesirably exhaust steam if the system overpressures due to improper temperatures or water levels that may occur during startup. Providing an appropriate pressure is also particularly important because the water within the system must be maintained in its liquid state in order to achieve the desired heat transfer and heat capacity characteristics. Furthermore, if a higher pressure can be properly maintained, a higher overall water temperature can be used, which results in a faster heat transfer to the tires and thus a faster curing time. As such, it is particularly desirable to be able to operate close to the overall mechanical pressure limits of the system. Such operation requires more precise control of temperature and pressure. More specifically, variations of temperature and pressure about the desired setpoints of the system must be minimized or even eliminated.
Accordingly, as will now be described, the present invention provides an automatic pressure and temperature control apparatus that allows for improved operation and control in the use of water as a heat transfer medium for curing tire assemblies.